1. Field of the Invention
The present invention relates to a magnetic alloy material that can be used effectively as a magnetic refrigerant material or a magnetostrictive material and also relates to a method of making such a magnetic alloy material.
2. Description of the Related Art
A magnetic alloy, having a composition represented by the general formula: La1-zREz(Fe1-xAx-yTMy)13 (where A is at least one element that is selected from the group consisting of Al, Si, Ga, Ge and Sn; TM is at least one of the transition metal elements; RE is at least one of the rare-earth elements except La; and the mole fractions x, y and z satisfy 0.05≦x≦0.2, 0≦y≦0.1 and 0≦z≦0.1, respectively, and which will be referred to herein as an “La(Fe, Si)13 based magnetic alloy”) has an NaZn13-type crystal structure and exhibits giant magnetocaloric effect and magnetovolume effect at temperatures around its Curie temperature Tc. The La(Fe, Si)13 based magnetic alloy is recently expected to be applicable for use as a magnetic refrigerant material or as a magnetostrictive material (see Patent Documents Nos. 1 and 2, for example).
In the prior art, the La(Fe, Si)13 based magnetic alloy is produced by thermally treating a mold-cast alloy, obtained by an arc melting or high frequency melting process, at 1,050° C. for approximately 168 hours within a vacuum, which results in very low productivity.
The applicant of the present application disclosed a method of making an La(Fe, Si)13 based magnetic alloy material highly efficiently by a melt-quenching process (which will also be referred to herein as an “rapid solidification process”) in Patent Document No. 3. However, if the magnetic alloy material disclosed in Patent Document No. 3 is used as a magnetic refrigerant material, then the magnetic refrigerant material should have its area of thermal contact with a heat transfer fluid increased by using an alloy material prepared by coarsely pulverizing a ribbon alloy material. The heat transfer fluid is preferably a liquid fluid including an aqueous antifreeze agent, having a relatively high specific heat and exhibiting good fluidity at its operating temperature, and a hydrocarbon based solvent with a low freezing point. And as the magnetic refrigerant material, a bed obtained by storing a coarse powder of an alloy material into a basket type container, a powder compact that has been compressed and compacted into thin plate shapes, and a sintered body that has been sintered into a porous bulk shape such that a liquid can pass through the body may be used.
Meanwhile, a method of making an La(Fe, Si)13 based magnetic alloy sintered body in a desired shape by a powder metallurgical process is described in Patent Document No. 4. In a powder metallurgical process, a sintered body is obtained by sintering a compact (i.e., a powder compact) that has been formed by pressing and compacting an alloy powder (fine powder). Thus, the powder metallurgical process needs an increased number of manufacturing process steps but realizes a broader variety of shapes with increased freedom. As a result, the processing cost can be rather reduced.
Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2000-54086,
Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2002-69596,
Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2004-100043 and
Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2005-36302
The present inventors attempted to apply a powder metallurgical process to processing an La(Fe, Si)13 based magnetic alloy material. As a result, we faced the following problems.
Specifically, to apply a powder metallurgical process to the La(Fe, Si)13 based magnetic alloy material disclosed in Patent Document No. 3, the target NaZn13-type compound phase needs to be produced by a heat treatment process, finely pulverized, and then a compact needs to be made of the resultant powder and sintered. That is to say, if the manufacturing process described in Patent Document No. 3 is adopted, then the overall heat treatment time can be shortened significantly. However, the heat treatment processes need to be carried out twice in a vacuum to produce the NaZn13-type compound phase and to sinter the compact, respectively, which should result in low productivity.
In addition, according to the method disclosed in Patent Document No. 3, the NaZn13-type compound phase can also be produced by a solid-phase reaction that is based on the element diffusion process in a ribbon of the as-spun alloy (i.e., rapidly solidified alloy). That is why even if the rapidly solidified alloy ribbon includes relatively coarse structures, the NaZn13-type compound phase can also be produced by thermally treating the rapidly solidified alloy ribbon. Nevertheless, if a powder metallurgical process is applied to such a rapidly solidified alloy including coarse structures, then the respective phases that form the structures may either be separate particles or have significantly different compositions between the particles. In that case, to produce the target phase, the element needs to transfer between the powder particles, thus requiring long hours of sintering (i.e., a type of heat treatment process), which is practically undesirable.
Additionally, in the as-spun state, it is usually very difficult to finely pulverize a structure in which Fe has grown into dendritic primary crystals with excessively large sizes. That is why even by adopting the rapid solidification process, if the size of the primary crystals of Fe is larger than a particle size (of 2 μm) required by the powder metallurgical process, it is extremely difficult to make a powder with the target particle size.
The alloy material described in Patent Document No. 4 does not have a sufficiently fine structure, either, because the alloy material is prepared at a low quenching rate of 1×104° C./s. Consequently, to make a sintered body consisting essentially of the NaZn13-type compound phase, (1) the proportion of the NaZn13-type compound phase to the overall material alloy needs to be increased to at least 85 mass % in advance by thermally treating the material alloy, (2) the sintering process should be carried out for long hours and (3) at as high a temperature as at least 1,280° C. and other problems arise.
If the material alloy has not been quenched so much (e.g., an ingot alloy), various problems also arise in the sintering process. Specifically, it is virtually impossible to eliminate the α -Fe phase at a sintering temperature that is lower than the peritectic point. At a temperature that is equal to or higher than the peritectic point, on the other hand, α -Fe phase, LaFeSi compound phase and other phases are newly produced. For these reasons, to make a single-phase, high-density sintered body, the sintering process needs to be carried out at an elevated temperature, precisely controlled within a narrow range, and for long hours.
In order to overcome the problems described above, a primary object of the present invention is to provide a method of making a sintered body, including an NaZn13-type compound phase, by a relatively cost-effective powder metallurgical process, which requires only a short sintering process time, and also provide a material alloy (powder) for use in the manufacturing process.